Wario

Wario is one of the main characters in the Mario series, even having his own WarioWare and Wario Land series. He first appeared in Super Mario Land 2: 6 Golden Coins as the main antagonist, though he didn't replace Bowser in the series. Different from other Mario characters, he is obese but strong with a greedy attitude. He sometimes teams up with his trickster friend (or brother), Waluigi, and has a romantic interest in one of the WarioWare characters, Mona.

You may have noticed a few changes in your former home. While you were off playing hero in Sarasaland, I took it upon myself to cast a spell over the population and turn the citizens into mindless zombies. I've also moved into the Castle on the hill, so I fear that you'll have to look for new accommodations. My entire operation is run from Wario Castle and the entrance is magically sealed. The only way to break into this fearsome fortress is to collect the Six Golden Coins and place them into the entrance. The Coins are hidden deep within the zones of the land and guarded by my army of oblivious devotees. Don't even think of trying to thwart me, Mario. It's Wario's World now and there's nothing that you can do to stop my sinister scheme.

Wario debuts as the antagonist in Super Mario Land 2: 6 Golden Coins. He turned all of the citizens of the Mushroom Kingdom into zombies and took Mario's castle, even taking the liberty to switch the "M" symbol on the castle, turning it upside down to look like a "W". As stated in the letter, he sealed the doors that could only be opened with 6 golden coins. He was eventually defeated, not even his upgraded form of Rabbit Wario being able to defeat the heroic plumber.